After a long journey, I've finally located the fabled Dr. Dobb's
articles written by John Bass detailing his MacSCSI solution for the
original Macintosh 128K & 512K utilizing the original 64K ROM. I have
published the PDF file at this link (Beware it is a 6MB file):

There is complete software driver documentation and the hardware seems
pretty straight forward. This could well be the start of a means to
equip original 64K ROM Macs with SCSI drives and/or flash drives to
replace the failing HD20 hard drives, something that both the Apple II
community and even the Lisa have enjoyed for several years now.
Hopefully, some of the members of the group with software programming
skills might step up to the challenge of helping to adapt the code to
some contemporary options.